Remove ValueString(), instead uses HexStr() directly.

The logic of the (looks like) optimaztion for input less than or equal to 4 bytes is actually problematic:
  1. The CBigNum::getint() method doesn't always reflect the exact content of the input, e.g. when it is larger than INT_MAX limit;
  2. The specifier "%d" doesn't work as expected, and could produce a neg. sign "-";
  3. Even changing the callee to CBigNum::getuint() and the specifier to "%x" don't work either, since the result may lose a "0" prefix due to the first byte of input is less than 0x10.

Signed-off-by: Huang Le <4tarhl@gmail.com>